Obtain the power factor in the electric system. Determine the power factor by noting the phase shift between the voltage and current signal waves of the system from an oscilloscope. The phase shift is a value expressed in degrees. Calculate its cosine, which is the power factor of the electrical system.
Read from the main meter the values of the voltage and current that flows through the system. If this information is not clearly stated, use a multimeter to measure both the current and voltage through the system. The oscilloscope can also serve the multimeter’s purpose, but when in use you should consider only the RMS values.
Multiply the RMS values of the voltage and current together and the product you obtain is the apparent power. The units for the product at this stage are Volt Amps (VA). For example, considering an RMS voltage of 110V, such as that supplied in the U.S., and an RMS current of 1.6A, multiplying the two you get the product as 176VA. This value is called the apparent power.
Multiply the apparent power by the power factor. For example, a power factor of 0.95 and considering your apparent power as 176VA, you end up with 167.2 watts as the effective power of the system. Each appliance in the system draws a different amount of current from the supply, therefore, it is possible to measure the active power for a particular appliance in a home rather than calculating it generally. Determine the current through the appliance, the voltage across it as well as the appliance’s power factor. Calculate the effective energy it consumes by multiplying all the three values together.